Benefits
Subjective energy after a single dose
In a University of Iowa crossover trial in 25 adults, a single 600 mg dose of AMATEA® Max (120 mg caffeine) raised perceived energy by 13 points and lowered fatigue-inertia and total mood disturbance versus placebo. Those are self-reported scales, and 120 mg of caffeine on its own would be expected to do much the same. The claim that guayusa avoids jitteriness is not supported by its own trials: in that study jitteriness rose with increasing dose in women (p < 0.001), and in a caffeine-controlled gaming trial jitteriness on AMATEA® was no lower than on an equal 270 mg dose of plain caffeine.
Cognitive performance and alertness
Against placebo, guayusa extract has improved objective measures: motor and psychomotor speed at 600 mg in 25 adults, and N-back reaction time, correct responses and Go-stimulus reaction time after 650 mg in 40 adults. In that second study the same dose also reduced the percentage of correct No-go responses, meaning worse response inhibition. Against a caffeine-matched control, the one trial that ran one found no significant cognitive advantage. The mechanism on offer is adenosine A1 and A2A antagonism by caffeine. There is no L-theanine counterweight: the one published analysis that tested guayusa for L-theanine found none.
High chlorogenic acid content, metabolic effects unmeasured
AMATEA® Max is 30 percent chlorogenic acids, so a 600 mg dose supplies about 180 mg, and guayusa leaf is a genuinely rich source of these polyphenols. What has not happened is any measurement of fat oxidation, insulin sensitivity, blood glucose or antioxidant status in a human trial of guayusa. The metabolic effects attributed to chlorogenic acids come from green coffee extract research at different doses in different preparations, so treat them as borrowed rather than demonstrated for this ingredient.
Caffeine delivered in a leaf extract
Guayusa leaf contains caffeine plus a trace of theobromine, reported at 0.02 to 0.12 percent of leaf weight, and theobromine does not appear at all in the published list of active compounds for AMATEA® Max. Theophylline is not listed among guayusa's methylxanthines in the composition review, and the one published analysis that tested guayusa for L-theanine found none. In a crossover study in 12 men, 200 mg of caffeine from guayusa extract and 200 mg of synthetic caffeine were bioequivalent for peak concentration and total exposure by FDA criteria, so the absorption curve is not slower or broader. The one measured difference was a smaller rise in urinary epinephrine after guayusa (+0.5 versus +2.78 micrograms per gram creatinine, p = 0.04), and whether that translates into a smoother subjective experience has not been shown.
Mechanism of action
Adenosine receptor antagonism (caffeine mechanism)
Caffeine from guayusa competes with adenosine at A1 and A2A receptors throughout the brain, blocking the fatigue-signaling cascade that adenosine normally triggers. This disinhibits dopamine and norepinephrine neurotransmission, producing the characteristic alertness, improved reaction time, and enhanced cognitive performance of caffeine — the world's most widely used psychoactive compound.
L-theanine has not been found in guayusa
The caffeine plus L-theanine combination has a real literature behind it, but it does not apply here. The 2020 critical review of guayusa's composition states that no published scientific study has identified L-theanine in guayusa, that L-theanine is otherwise known only from some Camellia species and one mushroom, that the vendor claims are either uncited or cite unrelated studies, and that the single peer-reviewed analysis to test guayusa for it found no detectable level. AMATEA® Max's own list of active compounds is caffeine, chlorogenic acids, quercetin glucosides and kaempferols, with no theanine. Anyone who wants the caffeine and theanine pairing has to take L-theanine separately.
Chlorogenic acid effects proposed from laboratory work
Chlorogenic acids inhibit glucose-6-phosphatase, inhibit alpha-glucosidase and activate AMPK in cell and animal work, and chlorogenic acid derivatives also bind adenosine transporters, which is the proposed reason guayusa might feel different from caffeine alone. None of this has been tested in a human trial of guayusa: no study of the extract has measured glucose, insulin or fat oxidation in people. Treat it as a hypothesis, not an effect.
Clinical trials
Helwig NJ, Schwager LE, Berry AC, Zucker AC, Venenga JS, Sterbenz SC, Jenkins NDM. Journal of the International Society of Sports Nutrition 2024;21(1):2379424. PMID 39014963. Funded primarily by Applied Food Sciences, the company that sells AMATEA®, which also manufactured, blinded and supplied the capsules. There was no caffeine-matched comparison arm, so the trial cannot separate guayusa from its caffeine.
25 healthy adults (9 men, 16 women, mean age 28) at the University of Iowa. Randomized, double-blind, placebo-controlled crossover; three visits with placebo, 600 mg AmaTea® Max (120 mg caffeine), or 1200 mg (240 mg caffeine). Single acute doses, all outcomes measured the same day.
600 mg improved total mood disturbance (-6.9 points), fatigue-inertia (-2.84), perceived energy (+13.0), motor speed (+4.52) and psychomotor speed (+7.20) versus placebo. 1200 mg improved psychomotor speed and cut reaction time on a neurocognitive hop test. On safety: both 600 mg and 1200 mg raised systolic and diastolic blood pressure by 4 to 5 mmHg (p <= 0.022), while heart rate and QTc were unchanged. Jitteriness was dose- and sex-dependent, rising with dose in women only (p < 0.001). The authors concluded that 600 mg suits mood and cognitive outcomes while limiting jitteriness in women, and that 1200 mg may be needed for motor-cognitive gains.
La Monica MB, Raub B, Ziegenfuss EJ, Hartshorn S, Grdic J, Gustat A, Sandrock J, Ziegenfuss TN. Nutrients 2023;15(24):5018. PMID 38140277. Run at the Center for Applied Health Sciences and funded in part by Applied Food Sciences, the AMATEA® supplier. Guayusa and Lion's Mane were separate arms; there was no combination arm and no caffeine-matched comparator.
40 healthy adults aged 18 to 50 who habitually consumed 240 mg of caffeine a day or less. Double-blind, randomized, three-arm, within-subject crossover: a single 650 mg dose of AmaTea® Max (130 mg caffeine, 30% chlorogenic acids), 1 g Nordic-grown Lion's Mane fruiting bodies, or maltodextrin placebo. Go/No-go, N-back and Serial 7s tasks plus subjective scales at baseline, 1 hour and 2 hours post-dose.
Guayusa improved N-back total score, correct responses, total responses and reaction time, improved Go-stimulus reaction time, and increased the number of attempts on Serial 7s. It also raised self-rated mental clarity, focus, concentration, mood and productivity at 1 and 2 hours. Against that, it reduced the percentage of correct responses to No-go stimuli, meaning more failures of response inhibition, and it was the only arm to raise blood pressure from baseline, by up to 5 to 6 mmHg systolic and 3 to 4 mmHg diastolic over 2 hours, although group averages stayed in the normotensive range. Two of the 40 participants reported dry mouth or tremor on guayusa. Lion's Mane separately improved Serial 7s attempts, N-back reaction time and Go-stimulus reaction time. Because the comparator was placebo rather than 130 mg of caffeine, this trial cannot show that guayusa did anything caffeine alone would not.